In men and male rhesus macaques testosterone (T) and dehydroepiandrosterone (DHEA, an adrenal androgen precursor) show characteristic 24-hour patterns in the circulation, and both show significant age-related decreases. Although the exact physiological consequence of these hormonal changes is unclear, both T and DHEA are thought to act as intracrine substrates for estradiol (E2) synthesis in the brain. Therefore, it is plausible that their age-related decline negatively impacts brain function, either directly through androgen receptors and/or indirectly through estrogen receptors. Using the rhesus macaque as a translational animal model, we propose to test the hypothesis that age-related attenuation of circulating T and DHEA levels negatively impacts centrally-mediated physiological processes, including the circadian sleep-wake cycle and cognition. Moreover, we predict that physiological testosterone supplementation, designed to mimic the circulating 24-hour T pattern of young animals, will ameliorate these age-associated disorders. Specific Aim 1 will use a battery of cognitive tests to assess differences between young and old male rhesus macaques, and between old animals receiving extended treatment with young physiological levels of T or placebo. Cognitive assessments will include: 1) the delayed response test of spatial working memory, which is particularly sensitive to aging and prefrontal cortex dysfunction; 2) delayed non-matching-to-sample, a task probing primarily temporal lobe-based memory; 3) a visuospatial cueing test of visual attention that is estrogen-sensitive, and 4) performance in a novel maze to assess spatial learning and memory. In addition, sleep-wake cycles will be continuously monitored using Actiwatches, while morphological and biochemical differences will be examined in targeted brain areas by magnetic resonance imaging (MRI). Specific Aim 2 will use a series of biochemical and histochemical methodologies to elucidate the plasticity that occurs within the central nervous system (CNS) during male aging and after supplementation with T. Rhesus-specific gene microarrays and quantitative real-time PCR will be used to identify genes that are differentially expressed in the CNS among young males, untreated old and the T-treated old males. This integrative systems approach should help to identify plasticity in neurotransmitter systems and synapses and shed light on potential regulatory mechanisms. In situ hybridization, immunohistochemistry, enzymology, and hormone measurements will be used to further corroborate the results. Our current NIH grant (AG-029612) similarly examines the interacting impact of ovarian-adrenal interactions in perimenopausal female rhesus macaques. Consequently, data from the ongoing female study, combined with data from the proposed male study, will disclose important gender-based differences and help to elucidate their underlying mechanisms.